update readme

Author: diveDylan

README.md

@@ -47,6 +47,7 @@ $ openapi --help
     --exportServices <value>  Write services to disk (default: true)
     --exportModels <value>    Write models to disk (default: true)
     --exportSchemas <value>   Write schemas to disk (default: false)
+    --apiAlias <value>        your api alias here
 
   Examples
     $ openapi --input ./spec.json